Cameron promised to deliver an in/out EU referendum by 2017, with the simple question "Should the United Kingdom remain a member of the European Union?" But his Conservative party is hoping to negotiate better terms and conditions for Britain's membership, rather than exiting the 28-nation bloc.

He is now hoping to drum up support for reform by visiting the bloc's 27 other leaders before the EU summit in June to deliver his side of the argument.

The Lisbon Treaty is the successor to the European Union Constitution, which became law in 2009. It includes heightened powers for the European Commission, European Parliament and European Court of Justice. It also opened the doors for more freedom of movement between EU members and greater control from Brussels over the final say on asylum applications.

In a press statement, the Prime Minister's office said that the choice for voters "should not be on the basis of the status quo but on a reformed relationship with the EU that the PM is determined to deliver".